1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More particularly, the present invention is in the field of fabrication of heterojunction bipolar transistors.
2. Related Art
GaAs based devices are able to provide the power and amplification requirements of various applications, such as wireless communication applications, with improved linearity and power efficiency. Of particular interest are gallium arsenide (xe2x80x9cGaAsxe2x80x9d) heterojunction bipolar transistors (xe2x80x9cHBTxe2x80x9d), which exhibit high power density capability, making them suitable as low cost and high power amplifiers in devices used in CDMA, TDMA and GSM wireless communications. However, GaAs HBTs, in particular GaAs HBTs utilizing an indium gallium phosphide (xe2x80x9cInGaPxe2x80x9d) emitter, can exhibit undesirable thermal instability, which can lead to catastrophic failure of the HBT.
By way of background, thermal instability can cause an HBT to self-destruct when increasing temperatures inside the HBT cause a decrease in the turn-on voltage of the base-emitter junction of the HBT. As the turn-on voltage decreases, the HBT turns on harder and thus consumes more power, which further increases the temperature in the HBT. The increasing temperature further decreases the base-emitter turn-on voltage, resulting in a positive feedback loop. This phenomenon, i.e. the creation of the positive feedback loop in the HBT due to the above-explained mechanism, is more specifically referred to as xe2x80x9cthermal runaway.xe2x80x9d Furthermore, as the overall temperature increases in the HBT, the internal areas of the HBT get hotter and, thus, carry more current than the periphery of the HBT. Accordingly, the temperature gradient causes the turn-on voltage in the internal base-emitter junction areas of the HBT to be lower than the turn-on voltage in the periphery areas of the base-emitter junction of the HBT. This results in filamentation, which occurs when localized current causes high power dissipation in a small area in an HBT with a correspondingly high increase in the localized temperature within the HBT. The end result of filamentation is self-destruction of the HBT. This phenomenon, i.e. filamentation and self-destruction of the HBT due to the above-explained mechanism, is more specifically referred to as xe2x80x9cthermal collapse.xe2x80x9d
In one known method to prevent the above positive feedback loop from occurring, an emitter ballast resistor is integrated into a GaAs HBT by adding a lightly doped epitaxial layer above a high band gap emitter. As the current in the GaAs HBT increases, the voltage across the epitaxial emitter ballast resistor increases which tends to decrease the voltage across the base-emitter junction of the HBT, thereby limiting the current flow into the GaAs HBT, which in turn stabilizes the HBT. For example, K. Yamamoto et al., xe2x80x9cA 3.2-V Operation Single-Chip Dual-Band AlGaAs/GaAs HBT MMIC Power Amplifier with Active Feedback Circuit Technique,xe2x80x9d IEEE Journal of Solid State Circuits, Vol. 35, No. 8, August 2000, pp. 1109-1120, discloses a lightly doped aluminum gallium arsenide (xe2x80x9cAlGaAsxe2x80x9d) ballast layer situated on top of a high band gap emitter. By way of further example, G. Gao et al., xe2x80x9cEmitter Ballasting Resistor Design for, and Current Handling Capability of AlGaAs/GaAs Power Heterojunction Bipolar Transistors,xe2x80x9d IEEE Transactions on Electron Devices, Vol. 38, No. 2, February 1991, pp. 185-196, discloses a lightly doped GaAs ballast layer situated on top of a high band gap emitter.
In a conventional epitaxial ballast resistor design, increased emitter resistance results mainly from two mechanisms: (1) the resistive nature of the low-doped ballast layer itself or (2) an increased thermionic emission barrier between the bottom of a low doped ballast layer, such as an emitter cap layer, and the top of a high band gap emitter. When a relatively thick layer is employed as an epitaxial ballast layer, mechanism (1) dominates, while mechanism (2) plays a significant role when the epitaxial ballast layer is relatively thin. In mechanism (1), increased emitter resistance critically depends on the doping level and thickness of the ballast layer while the emitter resistance will be mainly determined by the doping level of ballast layer in mechanism (2).
In both cases, i.e. in mechanisms (1) and (2), ballast resistance is very sensitive to the doping level of the epitaxial ballast layer and, consequently, requires very accurate doping control during the epitaxial growth process to achieve uniform, reproducible HBT characteristics. The above required accuracy in doping control creates manufacturing challenges that are difficult to meet. In addition, when mechanism (2) plays a significant role in determining total emitter resistance, emitter resistance tends to show increased base current dependency and a more negative temperature coefficient, which undesirably affects power amplifier linearity and thermal stability.
Referring now to FIG. 1, a conventional exemplary NPN GaAs HBT is illustrated. GaAs HBT 100 comprises emitter contact 120, base contacts 122 and 124, and collector contact 126. Further, GaAs HBT 100 comprises emitter cap 118, emitter cap 116, emitter 114, and base 112. In GaAs HBT 100, emitter cap 118 is indium gallium arsenide (xe2x80x9cInGaAsxe2x80x9d) grown with an N-type dopant such as tellurium at approximately 4xc3x971019 atoms per cm3, for example. Emitter cap 116 can be gallium arsenide doped with silicon at a relatively low doping level of approximately 5xc3x971018 atoms per cm3. Emitter cap 116 may have a thickness of approximately 2000 Angstroms. Emitter 114 can comprise either AlxGa(1-x)As or InxGa(1-x)P (referred to hereinafter simply as xe2x80x9cAlGaAsxe2x80x9d and xe2x80x9cInGaPxe2x80x9d) doped with silicon at a medium concentration of approximately 3xc3x971017 atoms per cm3. Base 112 can be, for example, gallium arsenide doped with carbon at a typical concentration level of approximately 4xc3x971019 atoms per cm3.
Continuing with FIG. 1, as shown, GaAs HBT 100 further comprises collector 130 and subcollector 110. According to conventional fabrication methods, collector 130 comprises gallium arsenide, which is uniformly and lightly doped with silicon at 1xc3x971016 atoms per cm3. Immediately below collector 130 is subcollector 110, which also comprises gallium arsenide. However, subcollector 110 is doped with silicon at a significantly higher concentration, typically in the range of approximately 5xc3x971018 atoms per cm3. In GaAs HBT 100, collector layer 130 can be between 0.3 microns and 2 microns thick, and subcollector 110 can be between 0.3 microns and 2 microns thick.
In previously known GaAs HBTs with an emitter ballast resistor, such as GaAs HBT 100, emitter cap 116 comprises relatively low silicon-doped gallium arsenide to provide an epitaxial emitter ballast resistor. As the current in GaAs HBT 100 increases, the voltage across the epitaxial emitter ballast resistor provided by emitter cap 116 increases which tends to reduce the voltage across the base-emitter junction of the HBT which, as discussed above, stabilizes GaAs HBT 100. Accordingly, emitter cap 116 can prevent a positive feedback loop from forming and destroying GaAs HBT 100, as discussed above. However, emitter cap 116 requires very accurate doping control during the epitaxial growth process to achieve uniform, reproducible HBT characteristics. The required accuracy in doping control of emitter cap 116 is very difficult to achieve at a relatively low silicon doping level, e.g. approximately 3xc3x971017 atoms per cm3, which is required to provide sufficient ballast resistance for GaAs HBT 100. Additionally, emitter ballast resistance provided by emitter cap 116 in conventional GaAs HBT 100 suffers from undesirable instability in resistance value as the base current is varied.
Another method utilizes an external emitter resistor to protect the GaAs HBT from destruction resulting from filamentation caused by the above positive feedback loop. However, since the resistance of the external emitter resistor is not distributed across the inner areas of the GaAs HBT, the external emitter resistor does not prevent filamentation from destroying the GaAs HBT.
There is thus a need in the art for an emitter ballast resistor in an HBT that provides a stable resistance that is not dependent on doping level.
The present invention is directed to structure and method for an emitter ballast resistor in an HBT. The present invention addresses and resolves the need in the art for an emitter ballast resistor in an HBT that provides a stable resistance that is not dependent on doping level.
According to one exemplary embodiment, a heterojunction bipolar transistor comprises an emitter. The heterojunction bipolar transistor, for example, may be an NPN GaAs heterojunction bipolar transistor. The heterojunction bipolar transistor further comprises a first emitter cap comprising a first high-doped layer, a low-doped layer, and a second high-doped layer, where the first high-doped layer is situated on the emitter, the low-doped layer is situated on the first high-doped layer, and the second high-doped layer is situated on the low-doped layer. The first high-doped layer, the low-doped layer, and the second high-doped layer form an emitter ballast resistor.
The resistance of the low-doped layer is dependent on the thickness of the low-doped layer, which may be approximately 900 Angstroms. The low-doped layer, for example, may be GaAs, InGaP, or AlGaAs, and may be doped with a silicon concentration of approximately 1xc3x971016 atoms per cm3.
The first high-doped layer and the second high-doped layer, for example, may be GaAs doped with a silicon concentration of between approximately 5xc3x971017 atoms per cm3 and approximately 1xc3x971019 atoms per cm3. The thickness of the first high-doped layer and the second high-doped layer, for example, may be approximately 150 Angstroms.
According to this exemplary embodiment, the low-doped layer has a thickness and a dopant concentration level such that the resistance of the low-doped layer is substantially independent of the dopant concentration level, but corresponds to the thickness of the low-doped layer. In another embodiment, the present invention is a method that achieves the above-described heterojunction bipolar transistor. Other features and advantages of the present invention will become more readily apparent to those of ordinary skill in the art after reviewing the following detailed description and accompanying drawings.